Yes Hoopster the hot and cold water must be off. Turn the boiler off, then the cold mains water. If you have a combi boiler (you will know this by listening to the boiler when you turn on a hot tap - If the boiler fires up; you have a combi) the hot will be automatically shut off when you turn off the cold.
If however you have tanked hot water,you will need to turn that off as well as the cold.
Look at our pages on hot and cold water systems to understand how your water system works. Then have a look at our page on changing taps. I hope this all helps.
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